In recent years, RFID systems utilizing a HF band of for example 13.56 MHz have been widely used as FeliCa (FeliCa: registered trademark) and near field communication (NFC) systems. For example, an antenna device for an RFID system is disclosed in Japanese Patent No. 3975918 (Patent Document 1).
FIG. 1 is a sectional view of an antenna device section of a communication terminal device 90 described in Patent Document 1. This antenna device includes an asymmetrically shaped coil in which the winding intervals at two positions that face each other across a central portion of the winding region of the coil 71 have been made to be different from each other. At a large winding interval side 71a, a magnetic body 72 is arranged on a surface of the coil 71 opposite to an input-unit-94-side surface, which faces an IC card 1, of the coil 71, and at a small winding interval side 71b, the magnetic body 72 is arranged on the input-unit-94-side surface, which faces the IC card, of the coil 71. A reader/writer circuit 50 is connected to the coil 71. The antenna device is arranged in an antenna housing concavity 97a formed in a metal casing 97 and is protected by a resin member 98. The magnetic field distribution of the antenna device is a distribution whose asymmetricity is more pronounced on the side 71a at which the winding interval and the line width of the coil 71 are larger. Consequently, an excellent communication state can be secured in a direction orthogonal to a main surface of the antenna device.